Conventional wind turbine towers typically include a tubular pole or a lattice structure to support a wind turbine at a considerable height to capture wind energy. The tubular pole configuration is relatively more simple and easier to assemble than the lattice structure. However, tubular poles use more steel than the lattice structure, resulting in a cost disadvantage with rising prices of steel. The lattice structure uses less steel, yet is relatively more complex due to numerous joints. These joints increase construction time and present possible locations for wear and maintenance. For example, vibration caused by wind against the wind turbine tower can loosen bolted connections over time. The bolted connections may be replaced with arc welded joints. Unfortunately, arc welded joints also may have drawbacks.
A known bolt connection used in lattice tower structures is depicted in FIGS. 4 and 5. With this configuration, the bolt 10 has a threaded end section 16 and a larger diameter chamfered shaft section 14. The threaded end section 16 is engaged by a washer 22 and nut 24 in the connected state of the bolt connection. The chamfered shaft section 14 lies between the bolt head 12 and the threaded end section 16 and engages in a relatively tight friction fit within aligned through holes 18 in the connected lattice components 20. Although this bolt configuration provides a relatively good balance between strength and maintenance, because of the length 26 of the interference fit between the chamfered shaft section 14 and through holes 18, there is little free length (pretension length) 28 left on the shaft to be stretched to achieve the nut pretension requirement. Lack of pretension may result in early failure of the nut contact and subsequent loosening of the bolt-nut combination.
Accordingly, an improved bolt connection configuration is desirable that provides the strength and maintenance aspects of the prior art bolt connection of FIGS. 4 and 5 while providing increased pretension length along the bolt shaft.